[Twenty Fifteen] customizer.php

I’d like to do two things.

1) This some code from the customizer.php file:

$colors = array(
		'background_color'            => $color_scheme[0],
		'header_background_color'     => $color_scheme[1],
		'box_background_color'        => $color_scheme[2],
		'textcolor'                   => $color_scheme[3],
		'secondary_textcolor'         => vsprintf( 'rgba( %1$s, %2$s, %3$s, 0.7)', $color_textcolor_rgb ),
		'border_color'                => vsprintf( 'rgba( %1$s, %2$s, %3$s, 0.1)', $color_textcolor_rgb ),
		'border_focus_color'          => vsprintf( 'rgba( %1$s, %2$s, %3$s, 0.3)', $color_textcolor_rgb ),
		'sidebar_textcolor'           => $color_scheme[4],
		'sidebar_border_color'        => vsprintf( 'rgba( %1$s, %2$s, %3$s, 0.1)', $color_sidebar_textcolor_rgb ),
		'sidebar_border_focus_color'  => vsprintf( 'rgba( %1$s, %2$s, %3$s, 0.3)', $color_sidebar_textcolor_rgb ),
		'secondary_sidebar_textcolor' => vsprintf( 'rgba( %1$s, %2$s, %3$s, 0.7)', $color_sidebar_textcolor_rgb ),
		'meta_box_background_color'   => $color_scheme[5],
	);

How can I add another entry to this array, say my_own_color?

2) How do I use that in $css function (it produces an inline css code)?